Frequency synthesizers are building block circuits in modern integrated transceivers. In order to meet phase noise requirements while consuming less power, inductance-capacitance voltage-controlled oscillators (LC-VCOs) are employed in frequency synthesizers. The inductance (L) in an LC-VCO circuit comes into existence when a current is made to flow through an inductor wire that encloses an area. To achieve inductance values having the high quality factors typically needed for modern transceivers, a significant silicon area is utilized. Other circuitry placed in the area enclosed by the inductor, causes degradation in the quality factor of the inductor because of eddy losses and consequently degrades the phase noise performance of the frequency synthesizer.
In copper based semiconductor manufacturing processes, a minimum density of metals for chemical mechanical processing (CMP) is needed in all layers, including the area enclosed by a frequency synthesizer inductor. Small pieces of metal are placed as a “density fill” within the enclosed inductor area to fulfill this minimum density requirement. These pieces of metal are small to reduce eddy losses and have no use other than meeting the metal density requirement for processing.